1. Field of the Invention
The invention relates to radio-frequency spectrum analysers using optical rf modulators for analysing signals and in particular though not exclusively to spectrum analysers employing Bragg cells in resonant cavities.
2. Discussion of Prior Art
One method for spectrum analysis of an FM signal is to apply the signal to a Bragg cell and measure the optical diffraction effects produced by the interaction of laser light and the acoustic waves present in the Bragg cell. The performance of Bragg cells in this application have the following performance limitations:
a) limited frequency bandwidth at high diffraction efficiency;
b) limited dynamic range; and
c) significant material intermodulation effects.
Steir et al, Applied Physics 27, No 8 (15 Apr. 1988), have shown that the diffraction efficiency of a stationary grating may be improved by placing the grating in a resonant cavity. The Bragg cell may be placed in a two-mirror resonant cavity, with one mirror 100% reflecting and the other mirror partially reflecting and this results in enhanced Bragg cell diffraction. This arrangement can be used in a white light RF spectrum analyser. The inventor has shown, however, that a practical spectrum analyser using monochromatic light cannot be based on this configuration. Another possible arrangement utilises a four mirror confocal cavity where the round trip path length for a ray of light remains constant for small changes in the Bragg angle. These arrangements have been shown to have limited bandwidth and resolution.